For late Renaissance astronomers the knowledge of the distance between the Earth and the Sun was a matter of utmost importance.
Since the assessment of Greek philosopher Hipparchus – and till the days of Kepler – the totally wrong figure of only 22 million kilometers was assumed.

Kepler himself had suggested to use the rare occasions of Venus transiting (crossing) the solar disk to measure our distance to the planet.
As stipulated by his 3rd. law of planetary motion the relative solar distances of the planets can be evaluated: this lead to the knowledge that Venus is 0.72 ( Earth = 1 ) Astronomical Units from the Sun.

But how to express this ratio into Kilometers or Miles ?
Two far away observers (one in Europe and the other one in the South Pacific for instance) will observe Venus under a certain angle during a transit against the solar disk due to their different line of sights.
By knowing this angle and the baseline seperating the observers a trigonometrical calculation will yield the distance to Venus. Even under the most favorable conditions however, this angle will never be greater than 1/50 of the solar diameter or 0.01°.
Such accuracy required precise timing of the event and exact knowledge of the terrestrial positions of the observers, a task difficult to master in these days.

Astronomical Odysseys

Due to the orbital inclination of Venus against the ecliptic of 3.5°, a Venus transit is so rare that it happens only at intervals of 121.5 years, 8 years, 105.5 years, 8 years and so on.
The first transit of Venus was observed in 1639 by J. Horrocks from England. The next transits were foreseen for 1761 and 1769.

The Royal Society of England and the French „Académie des Sciences „ made their preparations to send their astronomers to remote places of the rather unexplored world of these days.

The „Encyclopédistes“ of the Académie made their way to Siberia and the Southeast Pacific to observe the 1761 transit.
Jean Chappe left Paris in November 1760 for a 5000 km winter expedition over Strasbourg-Ulm-Vienna-Warsaw and then by sled over St. Petersburg to the Siberian town of Tobolsk.
After all these tribulations he could observe the entire transit.

On the other hand Le Gentil departed for a journey to India that would eventually become an odyssey of 11 years:
In the midst of the 7 years war between England and France he had to observe the transit from Isle de France.
Lacking precise clocks his observations were scientifically useless. He decided to wait instead without returning to France for the 1769 transit.
He became a tragic astronomer due a fatal cloud preventing any observations of the phenomena.

The most famous expedition of the 1769 transit was doubtlessly that one of James Cook to Tahiti in the South Pacific.
On August 26, 1768 his ship Endeavour sailed from Plymouth around Cape Horn to Tahiti. After concluding the transit observations from down there, they discovered New Zealand, charted The Great Barrier Reef and much more hitherto unexplored coasts ot Australia.
Only on July 13, 1771 – with half the crew having died from malaria and dysentery- the Endeavour went home.

Based upon the transit observations, French astronomer Lalande found in the year 1771 values of the Earth’s mean distance from the Sun ( the Astronomical Unit ) in the range of 152 to 154 Million Kilometers.
After the 19th. century transits of 1874 and 1882 Simon Newcomb determined a remarkably correct value of 149.59 Million Kilometers (nowadays value is 149.597870691 Million Kilometers).
Modern astronomy has evaluated this value by pumping powerful radio signals with giant radiotelescopes to Venus.
In conjunction with atomic clocks the roundtrip is precisely calculated by measuring when the faint echo from the planet is returning to Earth.

21st. Century Transits

The next transists of Venus will take place in the morning hours of June 8, 2004 (observable from Luxembourg) and June 5-6, 2012 (partially observable from Luxembourg ).
No single transit occurred in the whole 20th. century.